Let's pretend that gravity stopped. It is only make-believe, but it shows what gravity does for us. Gravity is the invisible pull that makes things fall down and keeps you on the ground. With no gravity, nothing would hold you down. When you throw something up, gravity pulls it back down. Without gravity, nothing would pull a thrown ball back.
Gravity holds down the air we breathe, so Earth keeps its blanket of air. Without gravity holding it, Earth would not keep that blanket. The Moon's gravity also pulls on the seas, and that makes the ocean tides go up and down. Without the Moon's pull, the seas would have no such tides.
Your weight is how hard gravity pulls on you, and the pull comes from all of Earth's mass. Your mass, the amount of stuff you are made of, stays the same even when your weight changes. A planet with less mass than Earth has weaker gravity, so you would weigh less there. On the Moon you would weigh about one-sixth of what you weigh on Earth, because the Moon pulls less. With no pull at all, you would have no weight.
Everything that has mass has gravity, and things with more mass pull harder. Gravity gets weaker the farther apart two things are. Albert Einstein described gravity as a curve in space that wraps around a star or a planet. Nearby objects are pulled into the curve. In 2015, scientists detected gravity's ripples, called gravitational waves, for the first time.
The Sun is a giant ball of hydrogen gas, and gravity holds all of that gas together. It would take almost 1,000 Jupiters to fill up the Sun. Gravity also keeps the Moon going around Earth, and Earth going around the Sun. The Sun's gravity keeps Earth at a comfortable distance for its light and warmth. Without gravity, nothing would hold the gas together or keep the Moon and Earth going around.
Everything in orbit is falling around its planet. It moves sideways so fast that it keeps missing the ground. The International Space Station is in low Earth orbit, and it circles the planet once in about 90 minutes. You may have heard that astronauts float because there is no gravity in space. In fact, gravity does not stop in space. They float because they are in free fall, and everything inside the station is falling together. Anything in free fall, like a dropped apple or a falling spacecraft, feels weightless because nothing is pushing back on it.
People on Earth can feel weightless too. On a roller coaster, free fall gives you a brief feeling of weightlessness at the top of a drop. To feel weightless for longer, people ride airplanes that fly in big arcs, so everyone inside floats during the fall. Gravity is never switched off in these flights.
Words to know
- gravity: invisible pull
- mass: amount of stuff
- orbit: path around something
- tides: rising falling seas
- gravitational: of gravity
